in this video we're going to focus on le chatelier's principle now you might be wondering what is le chatelier's principle what's the basic idea behind it well whenever you impose a change on a system at equilibrium the system is going to move in such a way as to undo that change or relieve some of that stress that you impose upon a system so let's say the system is at equilibrium let's say this line represents equilibrium let's draw a straight line now if you try to let's say raise it up let's say if you bring it up the system is going to try to bring it back down if you try to bring it down it's going to try to bring it back up whatever change you impose upon a system the system is going to try to undo that change and that's the basic idea behind lucitalia's principle now let's consider another example but one that deals with a chemical reaction so let's say we have reactant a and it's going to react with b b is going to be in the gaseous form and this is going to produce c which is a gas and d which is a liquid and let's say the coefficients are one two actually let's make this one we'll make this two and this will be one now this chemical reaction is at equilibrium equilibrium with regard to chemistry when a chemical reaction is in equilibrium it's not static equilibrium where everything is stationary we have a case of dynamic equilibrium that means things are moving when we have dynamic equilibrium the rate of the forward reaction is going to be equal to the rate of the reverse reaction because these two reactions are occurring at the same rate there's not going to be any net change in the amount of product and reactants so the concentration of the products and reactants they remain constant at equilibrium because the rate of the four reaction equals the rate of the reverse reaction so that's the situation that we have at equilibrium now let's consider what's going to happen if we add stress to the system so let's increase the concentration of reactant b if we introduce more of substance b to the vessel if we increase its concentration what's going to happen to this reaction will the reaction shift towards the right that is towards the products or will it shift towards the left towards the reactants so think of the concept of equilibrium we're increasing reactant b according to the shetalia's principle the system is going to try to undo the change that we have just done so since we've increased reactant b it's going to do something to decrease reactant b so in which direction should the reaction shift if we want to decrease reactant b if the reaction shifts to the right we know the products will increase if it shifts to the left the reactants will increase now we want the reactant to decrease not increase so it has to shift to the right whenever the reaction shifts to the right the reactants decreases the products increases when it shifts to the left the reactants they increase the products they decrease so we want the reactant to decrease therefore it's going to shift to the right now what happens if we decrease the concentration of reactant b let's say if we remove b from the reaction vessel what's going to happen now well since we brought b down according to the chatalia's principle this system is going to try to undo that change it's going to do the opposite of what you've done to the system so you brought the level of b down it's going to try to bring b back up so b is a reactant in order to increase the reactance we need to shift to the left so if you remove b from the reaction vessel the system is going to cause the reaction to shift to the left producing more b so whatever you do to the system it's going to try to undo the change that you impose upon it now let's focus on the products let's say if we increase the concentration of c what's going to happen will it shift to the right or to the left now c is a product and if we increase it the system is going to try to decrease it in which direction should the reaction shift in order to decrease the concentration of c or you could say the partial pressure of c since it's a gas if this system were to shift to the right we know the reactants would go down the products will go up c is a product the system is not trying to increase the product it wants to decrease it so it has to shift to the left where the reactants would go up the products will go down so if you increase c the system is going to move away from c to decrease it whenever the system moves away from a reactant or a product that product will react it's going to go down when the system moves towards it it's going to go up so let's say if we were to decrease c the system is going to try to increase c in order to increase in order for the system to increase the concentration of product c it has to shift towards the products so it's going to shift towards the right so to review if you were to increase the reactants the reaction is going to shift to the right if you were to increase the products the reaction is going to shift to the left now what's going to happen if we increase reactant a let's say if we introduce substance a or more of substance a into the reaction vessel what's going to happen to the reaction will it shift to the right or to the left a is in the solid state as a result increase in a will have no effect on the position of equilibrium the reaction will not shift to the right nor will it shift to the left now if you recall the equilibrium constant k is equal to the concentration of the products divided by the concentration of the reactants notice that a and d are not included in the equilibrium expression the equilibrium constant does not depend on the value of a or d because they're in a solid and liquid phase the only thing that will affect k is gases or substances in the gas phase or substances dissolve in the aqueous phase so if it's not dissolved in an aqueous solution or if it's not a gas it's not going to be included in the equilibrium expression so changing the value of a solid or liquid it's not going to have any effect on the position of equilibrium so if we were to increase d let's say if we were to add more of substance d this will have no effect on the position of equilibrium adding more liquid to the reaction vessel it's not going to affect these two reactants i mean it's not going to affect b or c because they're in the gas phase so they're outside of the liquid therefore to summarize increasing or decreasing any reactant or product in the solid or liquid phase will have no effect on a position of equilibrium so just keep that in mind but now let's go ahead and work on some practice problems number one which of the following actions will cause the reaction to shift toward the left so let's analyze each answer choice so what's going to happen if we increase the concentration of nitrogen gas nitrogen gas is a reactant if we increase the concentration of the reactant the system is going to try to undo that change it's going to try to bring the concentration of n2 back down and remember any time the reaction shifts towards the right the reactants will go down the products will go up if the reaction shifts to the left the reactants go up but the products go down n2 is a reactant so it's going to go in a direction to decrease that reactant so it's going to shift towards the right so thus we can eliminate answer choice a because it's going towards the right now what about b let's say if we were to remove nh3 from the mixture so if we're decreasing the concentration of nh3 in what direction will the reaction shift well according to le chatelier's principle we're decreasing the product the reaction is going to try to increase that product so in order to increase the product it has to shift to the right so we can also eliminate answer choice b since it's going to the right now what about c the addition of a catalyst if we were to add a catalyst what's going to happen would the reaction shift to the right or to the left the addition of a catalyst will cause the reaction to speed up so the reaction is going to go towards equilibrium at a faster rate a catalyst works by speeding up the rate of the forward reaction so it's going to increase kf but it also increases the rate of the reverse reaction so the reaction is going to go to the right at a faster rate but it's also going to go to the left at a faster rate thus it's not going to change the concentration of the products and the reactants because the relative concentration of the products and reactants won't change because there's no net change by introducing the catalyst the catalyst has no effect on the position of equilibrium even though it speeds up the reaction in both ways it doesn't shift it to the right nor does it shift it to the left so it has no effect so c is out so remember a catalyst simply speeds up a chemical reaction it has no effect on the position of equilibrium now what about removing hydrogen gas from the reaction vessel what's going to happen if we decrease the amount of hydrogen gas by taking it out of the reaction so hydrogen gas is a reactant if we decrease the amount of hydrogen gas the system is going to try to do the opposite it's going to try to increase so it's increasing a reactant now what must the system do in order to increase the reactant the only way it can increase the reactant is by shifting to the left and that's what's going to happen so if we remove h2 the system is going to try to produce more of h2 therefore d is the answer that we're looking for so that's the action that will cause the reaction to shift towards the left this by decreasing one of the reactants number two which of the following actions will cause the concentration of co to decrease in the reaction vessel so let's look at each answer choice if we were to add more ch4 or more methane to the reaction vessel what's going to happen as we increase the concentration of the product the reaction is going to shift to the left it's going to move away from the product side so as to decrease methane remember if it shifts to the left that is towards the reactants the reactants will go up it's moving away from the products so the products will go down as it shifts to the left what's going to happen to co what will be the effect of co now co is a reactant so when a reaction shifts to left co is going to increase methane being a product is going to decrease so since you increase methane the system is going to try to decrease it and as it shifts to the left the effect on cl is that it's going to increase so since cl is not decreasing a is not the answer we're looking for now what about b removing h2 from the reaction vessel so if we were to decrease the concentration of hydrogen gas what's going to happen hydrogen gas is a reactant and as you decrease hydrogen the reaction is going to shift towards it remember the system is going to try to do the opposite of what you did to it so because you added hydrogen the system wants to i mean because you removed hydrogen the system wants to increase it back to its original level or at least somewhere closer to its original level the only way it's going to increase the reactant is by shifting to the left which is what's going to happen and as it shifts to left the concentration of co is going to go up again so b is not the answer we're looking for it's not going to decrease the concentration of carbon monoxide now what about answer choice c what's going to happen if we increase the partial pressure of hydrogen gas now before we answer that question i want to mention a few things when speaking in terms of a reactant or a product that's dissolved in the aqueous solution the term partial pressure doesn't apply because it's not a gas however we can describe it in terms of its concentration but when dealing with the gases you can talk about it with reference to partial pressure since all gases exert some level of pressure but you can also talk about it in terms of concentration because in a reaction vessel there's going to be some number of moles of a particular gas in a certain volume the reaction vessel will have some amount of volume to it either 2 liter vesta or 3 liter vessel so you can describe a gas in terms of concentration and in terms of partial pressure now as we introduce more hydrogen gas to the reaction vessel the concentration of hydrogen goes up and the same is true for the partial pressure of hydrogen both goes up so the effect of increase in h2 will have the same effect as increase in the partial pressure of h2 they will cause the reaction to shift in the same direction so whether we're increasing the concentration of h2 or increasing its partial pressure the only way we can achieve that is by adding more h2 to the reaction vessel so what's going to happen as we add h2 as we increase the reactant the system is going to try to decrease the reactant it can only do that by shifting toward the products where the products will go up and the reactants will go down so it's going to shift to the right and as it shifts to the right it's moving away from co cl being a reactant so this time cl is going to decrease in value so that's how we can cause the concentration of co to decrease we need the reaction to shift to the right away from the reactants causing this to decrease while the products go up so answer choice c is the correct answer so here's a good way to remember this in a reaction we have the reactants on the left the products on the right now let's say the reaction is shifting towards the right notice that it's shifting towards the product side therefore the products should increase in value and it's shifting away from the reactant side therefore the reactants will decrease so that's a good way to remember what happens there when it shifts to the right when it shifts to the left notice that it's shifting toward the reactants so in whatever direction equilibrium shifts towards where the arrow points that's what's going to go up so as it shifts to the left toward the reactants the reactants is going to go up in value and it's shifting away from the product so the products will go down in value so that's a good way to remember what happens to the reactants and the products when the system either shifts to the right or to the left now let's look at answer choice d for the sake of understanding what happens if we were to add an inner gas such as neon i mean xenon rather to the reaction vessel neon is also an inner gas so this can be true of any one of the noble gases helium neon argon xenon krypton they're all inert gases what's going to happen if we add an inner gas to this reaction so if we were to introduce xenon xenon doesn't react with any of the reactants nor does it react with any of the products and it's not in this reaction it's not included in the equilibrium expression therefore the introduction of this inner gas will have no effect on the position of equilibrium the reaction will not shift to the right nor will it shift to the left so c is the correct answer for this problem number three which of the following statements is true if o2 is removed from the reaction vessel so if we decrease the concentration of o2 what will the system try to do to the concentration of o2 it's going to try to do the opposite it's going to try to increase it now in which direction will the reaction have to shift in order to increase o2 now o2 is a reactant so in order to increase the reactant it has to shift towards the reactant so the reaction has to shift towards the left as it shifts towards the left let me put this here the concentration of all reactants will go up when it shifts to left so so2 will increase in value and o2 will also increase in value which is what it's trying to do now the reaction is shifting away from the products so so3 is a product because the reaction is shifting away from it the product will go down in value so now we can find out which statement is true and which one is false so number one the reaction will shift to the right that's false we know it's going to shift to the left so statement two is true the concentration of so3 will increase that's false so3 is decreasing because it's going to shift away towards it i mean it's shifting away from so3 so that's why it's decreasing and number four the partial pressure of so2 will increase that's the true statement as the reaction shifts towards the left towards the reactant so2 being a reactant will increase in value so both the concentration and the partial pressure of so2 will increase so only statements 2 and 4 are true therefore answer choice d is the correct answer now let's move on to our next topic let's say we have a reacting with b to produce c and everything is in the gaseous phase what's going to happen if we increase the volume of the container will the reaction shift to the right or will it shift to the left now if we think about the ideal gas law pv is equal to nrt if we divide both sides by v we get that the pressure is equal to nrt over v notice that volume is in the denominator of that fraction that means that volume and pressure are inversely related if you expand the container if you increase its volume the pressure is going to go down and that is the total pressure of the system now what's going to happen if the total pressure decreases well the system is going to try to increase that total pressure whatever change you impose on the system the system wants to undo that change so if you decrease the total pressure of the system it's going to try to increase it now in which direction does the reaction have to shift in order to increase the total pressure should it shift to the left or to the right now notice that we have one gas molecule on the right side but we have two gas molecules on the left in order to increase the total pressure it needs to shift to the side with more moles of gas molecules so we have two moles of gas molecules on the right per one mole on the left so it's going to shift to the left side because there's more moles of gas and by shifting to the left it's going to increase the total pressure early we said that as we increase v the pressure decreases since v is on the bottom but n is in the numerator of that fraction as we increase n the pressure increases so in order to increase the total pressure we need to increase the total moles so when the system tries to increase the total pressure it's going to go to the side with more moles of gas there's a direct relationship between n and p now let's consider the reverse situation if we were to decrease the volume of the container the total pressure will increase now the system will try to decrease the total pressure so in order to decrease the total pressure it needs to decrease the number of moles there's a direct relationship between n and p and to decrease the number of moles it has to shift to the side with less moles of gas that is towards the right because there's less moles of gas there now let's understand more about this because we describe this in terms of the total pressure inside the container but let's talk about it in terms of the partial pressure so we're focused on the second situation as we decrease the volume the total pressure increases that means the partial pressure of every gas increases the partial pressure of a goes up the partial pressure of b goes up and the partial pressure of c goes up now what happens if we increase the partial pressure of a if you increase a the system is going to try to decrease it it's going to try to bring it down and the only way that's going to happen is if the system moves away from a so it's going to shift towards the right same thing for b if we increase the partial pressure of b the system is going to try to decrease it so it has to shift away from b to decrease the partial pressure b as we increase the partial pressure of c the system is going to try to do the reverse it wants to decrease it so it has to shift away from c that is towards the left so notice that we have two arrows going towards the right but only one arrow going towards the left the net effect is that these two is gonna win over this one so two against one the net result is that the reaction is going to shift towards the right if we decrease the volume you